In a Hydraulically-actuated Electronically-controlled Unit Injector fuel system, hereinafter referred to as a HEUI system, engine oil is drawn from the sump by the engine oil pump and flows to a high pressure supply pump which pressurizes the oil to between 450 and 3,000 psi. The outlet pressure of the high pressure supply pump is determined by the Rail Pressure Control Valve (hereinafter RPCV). High pressure oil from the RPCV is supplied to rails or manifolds that are cast into each cylinder head, from which it is constantly available to the fuel injectors. The pressure maintained by the RPCV is determined by a variable electrical current, hereinafter referred to as the duty cycle, that is supplied by the Electronic Control Module (hereinafter ECM). The ECM is a microprocessor which monitors various engine sensors and then computes a pair of output control signals. The ECM selects a fuel injector and sends an output signal to the Electronic Drive Unit (hereinafter EDU) which completes a circuit to the solenoid of the selected fuel injector. The energized fuel injector solenoid lifts the injector poppet valve off its seat which enables high pressure oil from the rail to enter the fuel injector causing injection of fuel into the engine cylinder. Injection of fuel stops when the ECM signals the EDU to stop transmitting the current to the injector solenoid. When this current stops, the popper spring then causes the poppet to close.
The actual rail pressure is sensed and transmitted to the ECM. The ECM utilizes the data from the various sensors to calculate a set point which is a calculated desired rail pressure. The difference between the set point and the actual rail pressure, is hereinafter referred to as the tracking error. The control strategy of the ECM includes lookup feed forward terms corresponding to tracking errors. The feed forward term is a first guess or calculation of the duty cycle needed to eliminate the tracking error. If the tracking error is not eliminated in the first cycle the cycle is repeated and a new duty cycle is calculated and transmitted to the solenoid of the RPCV which results in a further adjustment in the actual rail pressure. This closed loop system continues to recycle and to reduce the value of the tracking error. Recycling is time consuming and it is apparent that the accuracy of the look up feed forward term is critical to fast and accurate rail pressure adjustments.
An assumption of the control strategy, of the prior art HEUI system, is that the feed forward calibration is solely a function of the desired rail pressure. The ECM is programmed to be consistent with this assumption and selects a feed forward term accordingly. This assumption does not take into consideration the fact that engine operating conditions, such as operating speed and load, have a significant effect on RPCV flow and thus on the duty cycle required to attain a given rail pressure. Different operating conditions can require duty cycle swings of up to 3.5% for a given rail pressure.
Fast and accurate rail pressure control is essential to achieve optimum performance from HEUI fuel systems. However in prior art HEUI systems, for example, those disclosed in the above-identified U.S. Pat. Nos. 5,121,730 5,191,867 and 5,245,970, control tracking error has been observed when calibrating these engines especially under transient operating conditions. Tracking error, which is the difference between desired rail pressure and actual rail pressure, occurs when the control system cannot respond fast enough to accomplish the desired oil pressure change. Tracking error is affected by control system response time and the rail pressure information update rate. Tracking error adversely affects engine emissions, response time, fuel economy and performance.
For the foregoing reasons, there is a need to enhance the rail pressure control strategy used in HEUI systems, to improve the feed forward term and reduce the time required to reach a desired rail pressure.